Sodium dipotassium citrate, NaK2C6H5O7

The crystal structure of sodium dipotassium citrate has been solved and refined using laboratory X-ray powder diffraction data, and optimized using density functional techniques. The Na and K cation coordination spheres share corners and edges to form a three-dimensional network.

The crystal structure of sodium dipotassium citrate, Na + Á2K + ÁC 6 H 5 O 7 3À , has been solved and refined using laboratory X-ray powder diffraction data, and optimized using density functional techniques. The Na + and one of the K + cations are six-coordinate, with bond-valence sums of 1.13 and 0.92 valence units, respectively, while another crystallographically independent K + cation is seven-coordinate with a bond-valence sum of 1.20. The [KO 6 ] and [KO 7 ] polyhedra share edges and corners to form layers perpendicular to the b axis. The distorted [NaO 6 ] octahedra share edges to form chains along the a axis. The result is a three-dimensional network. The only O-HÁ Á ÁO hydrogen bond is an intramolecular one between the hydroxy group and a terminal carboxylate group.

Chemical context
We have carried out a systematic study of the crystal structures of Group 1 (alkali metal) citrate salts to understand the anion's conformational flexibility, ionization, coordination tendencies, and hydrogen bonding. Most of the new structures were solved using powder diffraction data (laboratory and/or synchrotron), but single crystals were used where available. The general trends and conclusions about the 16 new compounds and 12 previously characterized structures are being reported separately (Rammohan & Kaduk, 2016a). The initial study considered salts containing one type of Group 1 cations. This compound ( Fig. 1) represents an extension of the study to salts containing more than one alkali metal cation. The structure of related sodium potassium hydrogen citrate has been published recently (Rammohan & Kaduk, 2016b).

Structural commentary
The root-mean-square deviation of the non-hydrogen atoms in the refined and optimized structures is only 0.069 Å . The excellent agreement between the structures (Fig. 2) is strong evidence that the experimental structure is correct (van de Streek & Neumann, 2014). This discussion uses the DFToptimized structure. All of the bond lengths and torsion angles, and most of the bond angles fall within the normal ranges indicated by a Mercury Mogul Geometry Check (Macrae et al., 2008). Only the O17-C3-C4 [observed = 115.4 (4), optimized = 109.3, normal = 110.6 (3) , Z-score = 4.9] and O17-C3-C6 [observed = 109.0 (3), optimized = 111.4, normal = 105.4 (6) , Z-score = 10.5] angles are flagged as unusual. Part of the reason for the high Z-scores is the exceptionally low standard uncertainties on the normal values. The hydroxy group O17-H18 bridges Na19 and K20, so a small distortion from the normal geometry may be expected. The citrate anion occurs in the trans,trans-conformation (about C2-C3 and C3-C4), which is one of the two lowenergy conformations of an isolated citrate. The central carboxylate group and the hydroxy group occur in the normal planar arrangement. The citrate chelates to Na19 through the terminal carboxylate oxygen O12, the central carboxylate oxygen O17, and the hydroxy oxygen O17. The citrate chelates to K20 through the terminal carboxylate oxygen O12 and the hydroxy oxygen O17. One terminal carboxylate group (C1/ O11/O12) chelates to K21. Na19 is six-coordinate (distorted octahedral), with a bond-valence sum of 1.13 valence units (v.u.). K20 is also six-coordinate with a bond-valence sum of 0.92 v.u.; K21 is seven-coordinate, with a bond-valence sum of 1.20 v.u. Na19 and K21 are thus slightly crowded, while K20 is slightly underbonded. The metal-oxygen bonding is ionic, based on the cation charges and Mulliken overlap populations.

Supramolecular features
In the crystal structure (Fig. 3), the [KO 6 ] and [KO 7 ] polyhedra share edges and corners to form layers perpendicular to the b axis. The distorted [NaO 6 ] octahedra share edges to form chains along the a axis. The result is a three-dimensional network. The only O-HÁ Á ÁO hydrogen bond is an intramolecular one, O17-H18Á Á ÁO14 (Table 1), between the hydroxy group and a terminal carboxylate. Two intermolecular C-HÁ Á ÁO hydrogen bonds also apparently contribute to the crystal energy.

Database survey
Details of the comprehensive literature search for citrate structures are presented in Rammohan & Kaduk (2016a). A reduced cell search in the Cambridge Structural Database (Groom & Allen, 2014) (increasing the default tolerance from 1.5 to 2.0%, to account for the differences between ambient and low-temperature lattice parameters) yielded 25 hits, but limiting the chemistry to C, H, O, Na, and K only resulted in no hits. The powder pattern matched no entry in the Powder Diffraction File (ICDD, 2015).  Table 1 Hydrogen-bond geometry (Å , ) for the DFT-optimized structure.

Figure 2
Comparison of the refined and optimized structures of sodium dipotassium citrate. The refined structure is in red, and the DFToptimized structure is in blue.

Figure 3
Crystal structure of NaK 2 C 6 H 5 O 7 , viewed approximately down the a axis.

Figure 1
The content of asymmetric unit of the title compound showing the atom numbering and 50% probability displacement spheroids.
Aldrich) and 1.3824 g K 2 CO 3 (20.0 mmol K, Sigma-Aldrich) were added to the citric acid solution slowly with stirring. The resulting clear colorless colution was evaporated to dryness in a 393 K oven.

Refinement details
Crystal data, data collection and structure refinement details are summarized in Table 2. The powder pattern ( Fig. 4) was indexed using Jade 9.5 (MDI, 2012), which yielded a primitive triclinic unit cell with two formula units and with the lattice parameters as given in Table 2. Pseudovoigt profile coefficients were as parameterized in Thompson et al. (1987), and the asymmetry correction of Finger et al. (1994) was applied and microstrain broadening by Stephens (1999). The structure was solved with FOX (Favre-Nicolin & Č erný, 2002) using a citrate, Na, and two K as fragments. One of the 10 solutions (2 Â 10 6 moves, with a bump penalty with weighting factor = 50) yielded a much lower cost function than the others. All C-C and C-O bond lengths were restrained, as were all bond angles. The hydrogen atoms were included at fixed positions, which were re-calculated during the course of the refinement. The U iso parameters of C2, C3, and C4 were constrained to be equal, and those of H7, H8, H9, and H10 were constrained to be 1.3 times that of these carbon atoms.
The U iso parameters of C1, C5, C6, and the oxygen atoms were constrained to be equal, and that of H18 was constrained to be 1.3 times this value. The Bravais-Friedel-Donnay-Harker (Bravais, 1866;Friedel, 1907;Donnay & Harker, 1937) morphology suggests that we might expect platy morphology for sodium dipotas-sium citrate, with {001} as the principal faces. A 2nd-order spherical harmonic preferred orientation model was included in the refinement. The texture index was only 1.006, indicating that preferred orientation was not significant in this rotated flat-plate specimen. The powder pattern is included in the Powder Diffraction File as entry 00-065-1254.

Density functional geometry optimization
A density functional geometry optimization (fixed experimental unit cell) was carried out using CRYSTAL09 (Dovesi et al., 2005). The basis sets for the H, C, and O atoms were those of Gatti et al. (1994), the basis sets for Na and K were those of Dovesi et al. (1991). The calculation used 8 k-points and the B3LYP functional, and took about 41 h on a 2.8 GHz PC. The U iso parameters from the Rietveld refinement were assigned to the optimized fractional coordinates.

Figure 4
Rietveld plot for the refinement of NaK 2 C 6 H 5 O 7 . The red crosses represent the observed data points, and the green line is the calculated pattern. The magenta curve is the difference pattern, plotted at the same scale as the other patterns. The vertical scale has been multiplied by a factor of 10 for 2 > 51.0 . The row of black tick marks indicates the Bragg reflection positions for the phase.